Eosinophilic gastrointestinal disorders (EGIDs) are a group of diseases characterized by eosinophilic inflammation of the gastrointestinal tract. In the past 20 years, there has been a dramatic increase in the incidence of EGIDs, particularly eosinophilic esophagitis (EoE), but also eosinophilic gastroenteritis (EGE). EGID patients often have numerous food hypersensitivities, and the disease goes into remission with the institution of an amino acid based allergen-free elemental diet. In sum, this suggests that EGID is a food allergen driven eosinophilic inflammatory gut disease. In our previous work, we have demonstrated that IL-5+ Th2 cells are associated with EGID, but not anaphylactic forms of food allergy. We have further characterized these IL-5+ Th2 cells as more highly differentiated Th2 cells that are the product of multiple rounds of antigenic stimulus. In sum, these findings suggest that food allergen-driven IL-5+ Th2 cells are the major pathway driving eosinophilic gut inflammation in EGID. To facilitate both functional studies as well as their identification in tissue samples, we sought to develop a phenotypic marker of IL-5+ Th2 cells. Using microarray expression profiling, we have identified a phenotypic marker for IL-5+ Th2 cells. This marker identifies an IL-5 bright population whose frequency is tightly correlated to peripheral blood eosinophil count. These findings strongly suggest that these IL-5+ Th2 cells are driving blood eosinophilia. In a variety of assay systems, IL-5+ Th2 cells have greater functional activity than IL-5- Th2 cells. This indicates that the IL-5+ subpopulation may be the major Th2 population driving eosinophilic inflammation in EGID. We have also shown that Th2 cells, and in particular IL-5+ Th2 cells, are highly sensitive to molecular target of rapamycin (mTOR) inhibitors, such as rapamycin/sirolimus. Current work is focused on understanding the mechanism for this greater sensitivity of IL-5+ Th2 cells. These results suggest that mTOR inhibitors may have activity in treating allergic diseases. To test this hypothesis we have begun protocol 13-I-0094, a Phase I Open-Label Study of Sirolimus in Eosinophil-Associated Gastrointestinal Disorders. Retinoic acid (RA) is critical to mounting a healthy immune response and has known pro-Th2 effects. We have extended these latter findings to demonstrate that RA enhances the output of Th2 cells from allergen driven cultures and that most of this effect was limited to the IL-5+ Th2 subpopulation. Conversely, a RA receptor alpha antagonist inhibited IL-5+ Th2 cell proliferation. Similarly, IL-5 gene expression was augmented by RA and reciprocally inhibited by a RA receptor antagonist. These results suggest that RA receptor antagonism is a potential means to therapeutically target allergic inflammation. In collaborative work with Dr. Amy Klion's group in the NIAID Laboratory of Parasitic Diseases, we have examined the pathogenesis of a unique case of Epstein-Barr virus (EBV) associated lymphocytic hypereosinophilic syndrome (HES). This case was notable for EBV infection of a T cell receptor Vbeta 5.1 T cell clone, which demonstrated greatly increased IL-5 and IL-13 expression. Although limited to a single patient, these data demonstrate a newly described mechanism for HES, which may be applicable to other idiopathic forms of HES. Previous investigations of IL-5+ Th2 cells have been limited to the blood. To examine the role of IL-5+ Th2 cells in the genesis of local gut inflammation, the above phenotypic marker is being employed to determine if these IL-5+ Th2 cells are present in gut tissue from patients with EGID (clinical protocol # 10-I-0196, Immunopathogenesis of Food Allergy and Eosinophilic Gastrointestinal Disorder).